Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup_data

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hi,

Read this message in a fixed width text editor with a lot of columns.

On Wed, Dec 28, 2022 at 03:58:12PM -0800, H. Peter Anvin wrote:
> Glad you asked.
> 
> So the kernel load addresses are parameterized in the kernel image
> setup header. One of the things that are so parameterized are the size
> and possible realignment of the kernel image in memory.
> 
> I'm very confused where you are getting the 64 MB number from. There
> should not be any such limitation.

Currently, QEMU appends it to the kernel image, not to the initramfs as
you suggest below. So, that winds up looking, currently, like:

          kernel image            setup_data
   |--------------------------||----------------|
0x100000                  0x100000+l1     0x100000+l1+l2

The problem is that this decompresses to 0x1000000 (one more zero). So
if l1 is > (0x1000000-0x100000), then this winds up looking like:

          kernel image            setup_data
   |--------------------------||----------------|
0x100000                  0x100000+l1     0x100000+l1+l2

                                 d e c o m p r e s s e d   k e r n e l
		     |-------------------------------------------------------------|
                0x1000000                                                     0x1000000+l3 

The decompressed kernel seemingly overwriting the compressed kernel
image isn't a problem, because that gets relocated to a higher address
early on in the boot process. setup_data, however, stays in the same
place, since those links are self referential and nothing fixes them up.
So the decompressed kernel clobbers it.

The solution in this commit adds a bunch of padding between the kernel
image and setup_data to avoid this. That looks like this:

          kernel image                            padding                               setup_data
   |--------------------------||---------------------------------------------------||----------------|
0x100000                  0x100000+l1                                         0x1000000+l3      0x1000000+l3+l2

                                 d e c o m p r e s s e d   k e r n e l
		     |-------------------------------------------------------------|
                0x1000000                                                     0x1000000+l3 

This way, the decompressed kernel doesn't clobber setup_data.

The problem is that if 0x1000000+l3-0x100000 is around 62 megabytes,
then the bootloader crashes when trying to dereference setup_data's
->len param at the end of initialize_identity_maps() in ident_map_64.c.
I don't know why it does this. If I could remove the 62 megabyte
restriction, then I could keep with this technique and all would be
well.

> In general, setup_data should be able to go anywhere the initrd can
> go, and so is subject to the same address cap (896 MB for old kernels,
> 4 GB on newer ones; this address too is enumerated in the header.)

It would be theoretically possible to attach it to the initrd image
instead of to the kernel image. As a last resort, I guess I can look
into doing that. However, that's going to require some serious rework
and plumbing of a lot of different components. So if I can make it work
as is, that'd be ideal. However, I need to figure out this weird 62 meg
limitation.

Any ideas on that?

Jason